To secure the viewing of multimedia contents and subject this viewing to certain terms, such as taking out a paid subscription for example, the multimedia contents are broadcast in scrambled form and not in unencrypted or plain form. In this description, the channel is said to be “scrambled” when the multimedia content broadcast on this channel is scrambled.
More specifically, each multimedia content is divided into a succession of cryptoperiods. Throughout the duration of a cryptoperiod, the conditions of access to the scrambled multimedia content remain unchanged. In particular, throughout the duration of a cryptoperiod, the multimedia content is scrambled with the same control word. In general, the control word varies from one cryptoperiod to another.
Furthermore, the control word is generally specific to a multimedia content, this control word being drawn randomly or pseudo-randomly. Thus if at a given instant N multimedia contents are broadcast simultaneously on N channels, there are N different, independent control words, each used to scramble one of the multimedia contents.
Here, the terms “to scramble” and “to encrypt” are considered to be synonyms. This is also the case for the terms “to descramble” and “to decrypt”.
The plain multimedia content corresponds to the multimedia content before it is scrambled. This content can be made directly comprehensible to a human being without resorting to operations of descrambling and without subjecting the viewing to certain terms and conditions.
The control words needed to descramble multimedia contents are transmitted in synchronism with the multimedia contents. For example, the control words needed to descramble the tth cryptoperiod are received by each terminal during the (t−1)th cryptoperiod. To this end, for example, the control words are multiplexed with the scrambled multimedia content.
To secure the transmission of the control words, these words are transmitted to the terminal in a form of cryptograms contained in ECMs. Here below, the term “cryptogram” designates a piece of information that is not enough on its own to retrieve the unencrypted or plain control word. Thus, if the transmission of the control word is intercepted, knowledge of the cryptogram of the control word alone cannot be used to retrieve the control word by which the multimedia content can be descrambled.
To retrieve the plain control word, i.e. the control word that can be used to directly descramble the multimedia content, this control word must be combined with a piece of secret information. For example, the cryptogram of the control word is obtained by encrypting the plain control word with an operating key and an encryption algorithm. In this case, the piece of secret information is the operating key used and/or an encryption algorithm enabling the cryptogram to be decrypted.
The piece of secret information must be kept in a secure place. To this end, it has already been proposed to store the piece of secret information in security processors such as smart cards or again virtual cards. The term “virtual card” designates a software component comprising a set of resources, among them:                the executed code of an encryption algorithm or a decryption algorithm, and        the executable code of a syntax analyzer to locate a cryptogram of a control word within an ECM or the executable code of a syntax constructor to construct an ECM, and        as the case may be, access titles,        as the case may be, an operating key used as parameter of the encryption or decryption algorithm.        
An executable code is a code that can be directly executed by an interpreter or virtual machine at a lower level implemented in a microprocessor. The encryption or decryption algorithm and the syntax analyzer typically form an executable program or several executable programs.
Here below, the term “virtual mother card” refers to a virtual card used to compute an ECM. The term “virtual daughter” designates a virtual card used to process a received ECM. A virtual mother card and a virtual daughter card are said to be associated with each other if the virtual daughter card enables the successful processing of a received ECM, computed by means of the virtual mother card.
In this context, a method for transmitting and receiving a multimedia content, each cryptoperiod CPt of which is scrambled by means of a respective control word CWt, known to the filing party, comprises:                the encryption, by a sender, of the control word CWt by means of an operating key and a executable code of an encryption algorithm contained in the virtual mother card to obtain a cryptogram CWt*,        the generation of an ECM (Entitlement Control Message) incorporating the cryptogram CWt* by means of an executable code of a syntax constructor contained in the virtual mother card and the transmission of this ECM to a terminal,        the reception of the ECM by the terminal, the locating of the position of the cryptogram CW*t in the ECM received by means of an executable code of a syntax analyzer and then the decryption of the cryptogram CWt* by means of an operating key of a decryption algorithm, the executable code of the syntax analyzer and the decryption algorithm being contained in a virtual daughter card associated with the virtual mother card, and        the descrambling of the cryptoperiod CPt of the scrambled multimedia content by means of the decrypted control word CWt.        
The use of virtual cards enables the rapid and low-cost replacement of the secret information in the terminals. For example, the replacement of a virtual card enables the modification of the encryption and decryption algorithms used when a security breach has been discovered. However, the use of a virtual card in itself brings no gain in security as compared with the use of a smart card.
The prior art is also known from:                FR2922393A1,        EP1320006A1,        WO2009/112966A2, and        US2009/080648A1.        